The absolute frequency of a laser can be stabilized by locking the laser onto the transmission port of a resonator such as an optical ring or Fabry Perot (linear) resonator that has a stable resonance frequency. Using the transmission port (also referred to herein as “transmission mode”) of the resonator can have much lower resonance lineshape asymmetry than the resonator reflection port. Lineshape asymmetries in the reflection port can result from mode mismatches between the light that travels through the resonator and light that is rejected by the resonator. The mismatches can be in either polarization modes or spatial modes. These types of lineshape asymmetries do not exist in the transmission port since only light that travels through the resonator is present at the transmission port.
The tradeoff of locking the laser onto the transmission port is that propagation delays through the resonator impose a limit on the bandwidth (i.e. speed) of the laser locking loop. In many applications there is a desire not only to reduce absolute laser frequency fluctuations at low frequencies but also at higher frequencies. The reduction of laser frequency fluctuations at a particular frequency depends on the gain in the laser locking loop at that frequency. In many applications, to reduce absolute laser frequency fluctuations at higher frequency requires a laser locking loop bandwidth that cannot be achieved when using the transmission port of the resonator.
To achieve a higher bandwidth laser locking loop, the Pound-Drever-Hall (PDH) technique can be used. For the PDH technique (also referred to herein as “reflection mode”) the laser is locked to the reflection port of the resonator by using a phase modulation at a very high frequency. By using the PDH technique, fast changes in the laser frequency can be detected at the reflection port before any changes occur with the light traveling through the resonator. The reason for this is that fast changes in laser frequency or phase cause nearly immediate changes in the light reflected by the resonator, which interferes with the light coming out of the resonator. The interference produces nearly instantaneous changes in light intensity at the reflection port. This light at the reflection port can therefore be used to quickly identify changes in laser frequency or phase. The long propagation delay through the resonator no longer becomes a limit to laser locking loop bandwidth.